EP3364169A1 - Process gas analyser - Google Patents

Abstract

A process gas analyzer includes a wavelength tunable laser diode (7) and a detector (8) in two different measuring heads (5, 6) mounted on diametrically opposite sides of a plant part (1) having a process gas (2) carried therein.

In order to compensate for the influence of atmospheric oxygen in the measuring heads (5, 6) on the measurement of the oxygen content in the process gas mixture (2), a temperature sensor (23, 24) is arranged in at least one of the two measuring heads (5, 6); stored in a memory (26) of an evaluation device (19) at different temperatures and without process gas mixture (2) ambient reference spectra (27, 27 _2f ) of the ambient air contained in the two measuring heads (5, 6); the evaluation device (19) is further configured to select one of the stored ambient reference spectra (27 _2f ) as a function of the temperature detected by the temperature sensor (23, 24) or to calculate it from two stored ambient reference spectra (27 _2f ) and to subtract it from the currently detected absorption spectrum (18 _2f ) of the process gas mixture (2) and to determine the oxygen concentration in the process gas mixture (2) from the difference spectrum (30 _2f ).

Description

• The invention relates to a process gas analyzer for analyzing a process gas mixture conducted in a plant part,
• comprising a wavelength-tunable laser diode and a detector, which are separated from the interior of the installation part by windows in two different measuring heads mounted on diametrically opposite sides of the installation part, so that the light from the laser diode falls onto the detector after irradiation of the installation part,
• with a control device, which is designed to control the laser diode with a periodic current-time function, so that upon detection of the light, a time-resolved absorption spectrum of the process gas mixture is obtained, and
• with an evaluation device which is designed to determine the concentration of this gas component in the process gas mixture based on a selected absorption line of a gas component of interest in the detected absorption spectrum of the process gas mixture.

Such a process gas analyzer is from the EP 1 693 665 A1 or EP 3 104 163 A1 known.

In laser absorption spectroscopy, a sample gas mixture is irradiated with the light of a wavelength tunable laser diode and the concentration of a gas component of interest of the sample gas mixture is determined from the reduction in light intensity caused by the absorption of the light at the location of a selected absorption line of the gas component. In this case, the laser diode is periodically driven by a triangular or sawtooth-shaped current-time function (current ramp) in order to scan the absorption line of the gas component as a function of wavelength. To increase the measurement accuracy, the current-time function with a sinusoidal signal predetermined frequency f and amplitude modulated and the light at a higher harmonic frequency, usually twice the frequency 2f, phase-sensitive detected and evaluated (so-called wavelength modulation spectroscopy or WMS). The 2f signal component in the detected light or detector signal depends on the modulation ratio, ie on the ratio of the modulation amplitude to the width of the sampled absorption line. At low modulation amplitude, the 2f signal component is directly proportional to the second derivative of the detector signal. The evaluation can z. B. by fitting (curve fitting) of the expected in the ideal case course or profile of the demodulated detector signal to its actual course. From the measurement result obtained, the concentration of the gas component to be measured is finally determined.

For in-situ process gas analyzers, as stated in the above EP 1 693 665 A1 or EP 3 104 163 A1 are known, the laser diode and the detector are usually housed in different measuring heads, which are mounted on process flanges on diametrically opposite sides of a process gas mixture containing or leading the process gas mixture to be measured (eg., Exhaust pipe, tank, chimney). So that the laser diode and the detector do not come into contact with the often aggressive, hot and dusty process gas mixture, they are arranged behind windows. As the EP 1 693 665 A1 discloses the interiors of the measuring heads with a purge gas such. B. dry nitrogen to prevent the ingress of ambient air and so the impairment of the measurement, in particular trace gases, by gas components of the ambient air, such as water vapor. This leads to additional costs because the comparatively expensive purge gas has to be provided on the one hand and is consumed during the measuring operation on the other hand.

From the US 5,705,816 A or the US Pat. No. 6,483,589 B1 It is known the influence of ambient water vapor on the measurement of the concentration of water molecules or traces of water vapor be compensated in a measured gas mixture contained in a measuring cell by a part of the light coming from the laser diode coupled by means of a beam splitter from the optical path to the measuring cell and detected with an additional reference detector. The reference signal of the reference detector is instantiated by the measuring signal of the detector behind the measuring cell ( US 5,705,816 A ) or in the case of a spectrometer operating according to the WMS principle after 2f demodulation of both signals ( US Pat. No. 6,483,589 B1 subtracted. However, the additional optical surfaces of the beam splitter and the reference detector can lead to unwanted optical interferences that interfere with the measurement.

When measuring the oxygen content in a process gas mixture, the presence of atmospheric oxygen in the optical beam path within the two measuring heads represents a challenge.

The object of the invention is to compensate the influence of atmospheric oxygen in the measuring heads on the measurement of the oxygen content in the process gas mixture, without having to flush the measuring heads with an oxygen-free gas or to provide additional optical components in the light path from the laser diode to the detector.

According to the invention, the object is achieved in that in the process gas analyzer of the type specified in at least one of the two measuring heads, a temperature sensor is arranged that in a memory of the evaluation at different temperatures and without process gas mixture recorded ambient reference spectra of in the two Measuring sensors are stored ambient, and that the evaluation device is further adapted to select one of the stored ambient reference spectra depending on the temperature detected by the temperature sensor current or calculated from two stored ambient reference spectra and it from the currently detected absorption spectrum of the process gas mixture to subtract, then to a selected oxygen absorption line to determine the oxygen concentration in the process gas mixture.

Since the oxygen content of the atmosphere (about 20.9% Vol.) Is known and subject to minimal fluctuations, the wavelength-dependent absorption of light by the oxygen of the ambient air in the measuring heads before the analysis of the process gas mixture in the absence of the process gas mixture or any other oxygen-containing gas mixture between The measuring heads are determined and subtracted later in the analysis of the process gas mixture of the then measured absorption spectrum. The wavelength-dependent light absorption, ie the ambient reference spectrum, which takes place exclusively in the measuring heads, can be determined, for example, during the production of the measuring heads or before their mounting on the part of the system which carries the process gas mixture, with both measuring heads acting directly on their windows, ie. H. without space, abut each other or spaced from each other, wherein the space with an oxygen-free gas, for. B. nitrogen is filled. In order to determine the ambient reference spectrum under the same optical conditions as when using the gas analyzer in the process plant, the gap can be chosen to correspond to the internal cross section of the plant part to which the measuring heads are mounted. It is also possible to determine the ambient reference spectrum if the measuring heads are already mounted on the system part and the system is not in operation. In this case, the system part for recording the ambient reference spectrum would have to be rinsed once with a suitable oxygen-free gas.

The absorption spectrum of a gas or the selected absorption line for the analysis are temperature and pressure dependent, but the influence of changes in air pressure at one and the same location is negligible. There are therefore, z. B. in a climate chamber, ambient reference spectra recorded at different temperatures and stored together with these or these assignable. In the later Measuring operation is then selected on the basis of the currently measured temperature, the associated ambient reference spectrum or in the case of intermediate temperature values from two stored ambient reference spectra z. B. calculated by averaging or interpolation and subtracted from the currently detected absorption spectrum of the process gas mixture.

Since different temperatures can prevail in the two measuring heads due to their different mounting positions, if z. B. one of the measuring heads in sunlight and the other is in the shade, may be arranged in each case a temperature sensor in an advantageous manner in two measuring heads. The ambient reference spectra are then recorded and stored at different combinations of temperatures in the measuring heads and selected in the later measuring operation on the basis of measured in both measuring heads currently measured or, as explained above, calculated and subtracted from the currently detected absorption spectrum of the process gas mixture.

To reduce the effort involved in recording the environmental reference spectra, they can alternatively be determined and stored as a function of the temperature measured in only one measuring head, the ambient temperature being selected or calculated depending on the actual temperature detected in this one measuring head in the later measuring operation Reference spectrum is corrected as a function of the difference between the detected in both heads current temperatures with an empirically determined factor before it is subtracted from the currently detected absorption spectrum of the process gas mixture. This is especially possible if the temperature difference in the two measuring heads is rather low and z. B. limited to a few 10 degrees Kelvin.

As already mentioned, the influence of air pressure fluctuations on the measurement is negligible. In contrast, air pressure is more dependent on the altitude of the site above sea level. Because of this, the Ambient reference spectra additionally recorded at different ambient pressures and stored together with these or these assignable. For this purpose, the measuring heads can be flushed actively with ambient air with adjustable flushing pressure. In the later measuring operation, the ambient reference spectrum to be subtracted from the currently detected absorption spectrum of the process gas mixture is additionally selected or calculated as a function of the currently measured ambient pressure.

In the simplest case, the ambient reference spectra obtained directly from the detector, possibly subsequently amplified or filtered, can be stored and subtracted from the correspondingly obtained absorption spectrum during measuring operation. In the case of a process gas analyzer operating according to the WMS principle, in which the current-time function for driving the laser diode is modulated with a sinusoidal signal of predetermined frequency and modulation amplitude and the detected light is demodulated at the predetermined frequency or one of its higher harmonics, For example, it is preferably the ambient reference spectra obtained after demodulation which are stored and subtracted from the demodulated absorption spectra obtained during the measurement operation.

The ambient reference spectra to be subtracted from the absorption spectra obtained during the measurement operation can be calculated or simulated on the basis of comparatively less measured ambient reference spectra. As mentioned above, in wavelength modulation spectroscopy, the demodulated detector signal may be evaluated by sampling a parametrically writable profile of a demodulated ideal absorption line, one of the variable parameters being the concentration of the gas component of interest. Accordingly, such a profile can also be fitted to the oxygen absorption line in the measured ambient reference spectra. In this case, the detection of the ambient reference spectrum at different temperatures and pressures on comparatively few measurements and points be limited in order to calculate from the profile obtained by the fitting the profiles of the demodulated oxygen absorption line of the ambient air contained in the two measuring heads for virtually any temperatures and pressures. In the measuring operation, the profile of the demodulated oxygen absorption line of the ambient air is then calculated as a function of the currently measured temperature and possibly the measured pressure or selected from precalculated profiles and subtracted from the demodulated absorption spectrum.

A problem is a possible aging of the laser diode in the period between the acquisition and storage of the ambient reference spectra and a subsequent measurement of the process gas mixture, with the result that the optical power decreases and the wavelength of the light generated and thus the tuning behavior the laser diode changed over the current-time function. This can lead to the fact that the absorption spectrum obtained in the measurement no longer corresponds in terms of its width to the ambient reference spectrum to be subtracted by it, so that the further evaluation following the subtraction is faulty.

Such as B. from the EP 1 544 604 A1 or off P. Kluczynski et al .: "Wavelength modulation absorption spectrometry - an extensive scrutiny of the generation of signals" in Spectrochimica Acta Part B 56, August 2001, pages 1277-1354 , the profile of an absorption line demodulated at the Nth harmonic of the modulation frequency f can be described analytically, the description also including modulation-specific parameters associated with aging of the laser diode in addition to parameters describing the temperature and pressure dependence of the absorption line can change. These include the central wavelength or the inverse optical frequency for scanning the absorption line, the modulation amplitude of the wavelength or the inverse optical frequency and one or more modulation coefficients which the linear and possibly non-linear ratio between the modulation amplitude of the light intensity and the modulation amplitude of the wavelength or optical frequency.

Upon matching the profile of the demodulated ideal absorption line to the oxygen absorption line of interest in the recorded demodulated ambient reference spectrum, the modulation-specific parameters are given values dependent upon the state of the laser diode at the time the ambient reference spectrum is acquired. If, later on at the time of the current measurement, the profile of the demodulated ideal absorption line at an absorption line different from the oxygen absorption line of interest, e.g. B. another gas component of the process gas mixture, anfittenet in the detected and demodulated absorption spectrum, one obtains for the modulation-specific parameters new values that are dependent on the state of the laser diode at the time of the current measurement. With these new values, the profile of the demodulated oxygen absorption line of the ambient air which is selected as a function of the current temperature and optionally the current pressure and to be subtracted from the demodulated actual absorption spectrum can now be corrected as if it were with the laser diode in the current current state Aging condition would have been included.

Figur 1

ein Beispiel für einen in-situ Prozess-Gasanalysator,

Figur 2

ein Ausführungsbeispiel des erfindungsgemäßen Prozess-Gasanalysators,

Figur 3

ein Beispiel für die Subtraktion eines UmgebungsReferenzspektrums von dem aktuellen Absorptionsspektrum und

Figur 4

ein Beispiel für den Fehler, der entsteht, wenn das aktuelle Absorptionsspektrum und das von ihm zu subtrahierende Umgebungs-Referenzspektrum bei unterschiedlichen Alterungszuständen der Laserdiode aufgenommen worden sind.

To further explain the invention, reference will be made below to the figures of the drawing; in detail show:

FIG. 1

an example of an in-situ process gas analyzer,

FIG. 2

an embodiment of the process gas analyzer according to the invention,

FIG. 3

an example of the subtraction of an environmental reference spectrum from the current absorption spectrum and

FIG. 4

an example of the error that arises when the current absorption spectrum and the environmental reference spectrum to be subtracted by it have been recorded at different aging states of the laser diode.

FIG. 1 shows a schematic representation of a system part 1, z. B. an exhaust passage through which a process gas mixture 2 to be analyzed flows. The plant part 1 has at two diametrically opposite points process flanges 3, 4, on which two measuring heads 5, 6 of a process gas analyzer are mounted. Both measuring heads 5, 6 each contain an optoelectronic element 7, 8, which in one case is a laser diode 7 and in the other case a detector 8. The light 9 generated by the laser diode 7 is passed through the plant part 1 with the process gas mixture 2 guided therein and then impinges on the detector 8. The optoelectronic elements 7, 8 are through windows 10, 11 of the interior of the plant part 1 and thus of the Process gas mixture 2 separated

FIG. 2 shows an example of a block diagram of the process gas analyzer according to the invention. The laser diode 7 is driven by a controllable current source 12 with an injection current i (t), wherein the intensity and wavelength of the generated light 9 depend on the current i (t) and the operating temperature of the laser diode 7. The current source 12 is driven by a first signal generator 13 periodically with a predetermined, preferably ramped or triangular signal 14 to drive the laser diode 7 with a similar current-time function i (t) and with the correspondingly modulated light 9 a selected absorption line a gas component of interest, here oxygen, in the process gas mixture 2 to scan wavelength dependent. A second signal generator 15 generates a sinusoidal signal 16 of the frequency f, with which the ramped or triangular signal 14 is modulated in a summing element 17.

When irradiating the plant part 1, a small part of the light 9 generated by the laser diode 7 is absorbed by the different gas components of the process gas mixture 2 depending on the wavelength. As a function of the detected light intensity, the detector 8 generates a detector signal 18 which contains as information a time-resolved absorption spectrum of the process gas mixture 2 including a selected oxygen absorption line, by means of which the oxygen concentration in the process gas mixture 2 is determined in an evaluation device 19. For this purpose, the possibly amplified and filtered detector signal or absorption spectrum 18 is first in a demodulator 20 at the n-th, here z. B. the second harmonic 2f of the modulation frequency f demodulates. A computing device 21 further processes the obtained demodulated absorption spectrum 18 _2f at the location of the selected oxygen absorption line to a measured value 22 for the oxygen content of the process gas mixture 2.

In each of the two measuring heads 5, 6 in each case a temperature sensor 23 or 24 for measuring temperature T ₅ and T _{6 is arranged} in the respective measuring head. Optionally, a pressure sensor 25 for measuring the air pressure may be arranged in one of the two measuring heads, here the measuring head 6.

The measurement of the oxygen content in the process gas mixture 2 is superimposed by the atmospheric oxygen, which is located in the optical beam path within the two measuring heads 5, 6, ie between the laser diode 7 and the window 10 and between the window 11 and the detector 8. In order to compensate for the influence of the atmospheric oxygen on the measurement of the oxygen content in the process gas mixture 2, environmental reference spectra 27 _2f of the ambient air contained in the two measuring heads 5, 6 are stored in a memory 26, which are unique and without the presence of the process gas mixture 2 at different temperatures T ₅ , T ₆ and possibly different pressures P have been recorded. In the later measuring operation, the associated stored ambient reference spectrum is then selected on the basis of the currently measured temperatures T ₅ , T ₆ and possibly the pressure P, or in the case of intermediate temperature and / or pressure values are calculated by averaging or interpolation from two stored environmental reference spectra and subtracted from the currently detected absorption spectrum 18 _{2f of} the process gas mixture 2 in a subtractor 28 before the concentration of oxygen in the process gas mixture 2 is determined in the computing device 21. Since the subtraction takes place behind the demodulator 20, the stored ambient reference spectra 27 are the demodulated environmental reference _spectra 27 _2f . Alternatively, the subtraction may also take place before demodulation.

A communication link 29 allows in addition to the transmission of control and synchronization signals between the two measuring heads 5, 6 also z. B. the transmission of the temperature measured value T ₅ from the measuring head 5 in the measuring head. 6

Fig. 3 shows an example of the absorption spectrum 18 _{2f of} the selected oxygen absorption line obtained and demodulated in a current measurement, the 2f ambient reference spectrum 27 _{2f of} the atmospheric oxygen in the measuring heads 5, 6 to be subtracted therefrom and the 2f difference spectrum 30 obtained after the subtraction _2f , which corresponds to the absorption by the oxygen in the process gas mixture 2. I _2f denotes the intensity and 1 / λ the wave number. In the example shown, the oxygen content in the process gas mixture 2 is 1000 ppm, the optical measuring section in the plant part 1 is 2 m, the absorption distances in the measuring heads 5, 6 amount to 20 cm in total.

The 2f-ambient reference spectrum 27 _{2f to} be subtracted can also be the profile of an ideal demodulated absorption line, which is described by different parameters and by varying the parameters to that at certain temperatures T ₅ , T ₆ and at a certain pressure P demodulated environmental reference spectrum obtained by measurement has been fitted. From such profiles obtained by fitting can provide other profiles for other temperatures and pressures are calculated without requiring measurements of the environmental reference spectra.

und $$I_L\left(v_c,v_a,t\right)=I_{L,0}\left(v_c\right)+{\kappa }_1{v}_a\cos \left(2\mathit{\pi ft}+{\mathrm{\Phi }}_1\right)+{\kappa }_2{v}_a^2\cos \left(2\pi 2\mathit{ft}+{\mathrm{\Phi }}_2\right)\mathrm{.}$$

Such as B. from the above-mentioned publication P. Kluczynski et al .: "Wavelength modulation absorption spectrometry - an extensive scrutiny of the generation of signals" known, vary due to the sinusoidal modulation of the laser current i (t) with the frequency f, the optical frequency ν _L, and the intensity I _L of the generated light with 9: $$v_L=v_c\left(t\right)+v_a\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(2\mathit{\pi ft}\right)$$

and $$I_L\left(v_c,v_a,t\right)=I_{L.0}\left(v_c\right)+{\kappa }_1{v}_a\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(2\mathit{\pi ft}+{\mathrm{<figure-callout\; id="1"\; label="\Phi "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Phi </figure-callout>}}_1\right)+{\kappa }_2{v}_a^2\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}\left(2\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\pi </figure-callout>}2\mathit{ft}+{\mathrm{<figure-callout\; id="2"\; label="\Phi "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\Phi </figure-callout>}}_2\right)\mathrm{,}$$

In this case, ν _c denotes the optical center frequency around which the modulation takes place, ν _a the modulation amplitude of the optical frequency, I _{L, 0} the intensity of the generated light 9 at the optical center frequency, κ ₁ , κ ₂ a linear and non-linear modulation coefficient, which indicate linear and quadratic relationship between the modulation amplitude of the light intensity I _L and the modulation amplitude optical frequency ν _a , and Φ ₁ , Φ ₂ a phase angle between the modulation of the light intensity I _L and the modulation of the optical frequency ν _a .

After absorption of the light by an ideal absorption line with Lorentz profile X, the detected light intensity S _AS is obtained on the basis of the 2f demodulation of the detected light for the second harmonic even Fourier coefficient. $$S_{\mathit{AS}.2}^{\mathit{even}}\left(v\right)\approx -\mathit{\eta T}\cdot \left({\tilde{\chi }}_2^{\mathit{even}}\cdot I_{L.0}\left(v_c\right)+\frac{{\tilde{\chi }}_1^{\mathit{even}}+{\tilde{\chi }}_3^{\mathit{even}}}{2}\cdot {\kappa }_1{v}_a{\mathrm{<figure-callout\; id="1"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}}_1+{\tilde{\chi }}_0^{\mathit{even}}\cdot {\kappa }_2{v}_a^2{\mathrm{<figure-callout\; id="2"\; label="cos"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cos</figure-callout>}}_2+...\right)\cdot {\mathrm{<figure-callout\; id="0"\; label="\alpha "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\alpha </figure-callout>}}_0\mathrm{,}$$

Where η is a device factor, T is the wavelength-independent optical transmission, X _{n is} the n-th Fourier coefficients of the wavelength-modulated Lorentz profile and α _{0 is} the absorption at the center wavelength of the absorption line.

The parameters ν _c , ν _a , κ ₁ , κ ₂ can change as a result of aging of the laser diode 7, wherein the optical center frequency ν _c can be kept stable in a known manner by line locking.

FIG. 4 shows an example of the error that arises when the current absorption spectrum and the ambient reference spectrum to be subtracted from it have been recorded at different aging states of the laser diode 7. 31 _2f denotes the erroneous 2f signal obtained after the subtraction, while 32 _{2f is} the result of attempting to match the profile of an ideal demodulated absorption line to the erroneous 2f signal 31 _2f . For comparison, the demodulated absorption Line 30 is _2f of oxygen (1000 ppm) in the process gas mixture 2, which would have been obtained without the error after the subtraction.

By, in the current measurement, the profile, for example the Voigt profile or the Lorentz profile X, of a demodulated ideal absorption line to an absorption line different from the oxygen absorption line of interest, e.g. B. another gas component of the process gas mixture 2 or a measured in a reference channel gas component is fitted in the detected and demodulated absorption spectrum, one obtains for the modulation-specific parameters mentioned above ν _a , κ ₁ , κ ₂ values, the state of the laser diode. 7 at the time of the current measurement. These values can now be applied to the profile 27 _2f selected for subtracting from the demodulated actual absorption spectrum 18 _2f , in order to correct this according to the current state of aging.

Claims (7)

Process gas analyzer for analyzing a process gas mixture (2) conducted in a plant part (1), - having a wavelength - tunable laser diode (7) and a detector (8), which in two different, on diametrically opposite sides of the plant part (1) mounted measuring heads (5, 6) from the interior of the plant part (1) through windows (10, 11 ) are arranged separately, so that the light (9) of the laser diode (7) after irradiation of the plant part (1) on the detector (8) falls, - With a control device (12, 13, 15, 17) which is adapted to the laser diode (7) with a periodic current-time function (i (t)) to control, so that in the detection of the light (9) a time-resolved absorption spectrum (18) of the process gas mixture (2) is obtained, and with an evaluation device (19) which is designed to determine the concentration of this gas component in the process gas mixture (2) on the basis of a selected absorption line of a gas component of interest in the detected absorption spectrum (18, 18 _2f ) of the process gas mixture (2), characterized, that a temperature sensor (23, 24) is arranged in at least one of the two measuring heads (5, 6), - that in a memory (26) of the evaluation device (19) at different temperatures and without process gas mixture (2) recorded ambient reference spectra (27, 27 _2f ) of the two measuring heads (5, 6) contained ambient air are stored, and in that the evaluation device (19) is further configured to select one of the stored ambient reference spectra (27, 27 _2f ) as a function of the current temperature detected by the temperature sensor (23, 24) or from two stored environmental reference spectra (27, 27 _2f ) and to subtract it from the currently detected absorption spectrum (18, 18 _2f ) of the process gas mixture (2), and then to use a selected one Oxygen absorption line to determine the oxygen concentration in the process gas mixture (2). Process gas analyzer according to claim 1, characterized in that a temperature sensor (23, 24) is arranged in both measuring heads (5, 6) such that the ambient reference spectra (27, 27 _2f ) at different combinations of temperatures in the measuring heads ( 5, 6) and stored in the memory (26) and that the evaluation device (19) is adapted to the ambient reference spectrum (27, _{32, 18f} ) of the process gas mixture (2) to be subtracted from the currently detected absorption spectrum (18, 18). 27 _2f ) as a function of the actual temperatures measured in both measuring heads (5, 6) or to calculate. Process gas analyzer according to claim 1, characterized in that in both measuring heads (5, 6) each have a temperature sensor (23, 24) is arranged and that the evaluation device (19) is adapted to that with one of the two temperature sensors (23, 24) detected current temperature selected or calculated ambient reference spectrum (27, 27 _2f ) with an empirically determined factor as a function of the difference between the two temperature sensors (23, 24) detected actual temperatures correct. Process gas analyzer according to one of the preceding claims, characterized in that a pressure sensor (25) measuring the ambient pressure is present, that the ambient reference spectra (27, 27 _2f ) are additionally recorded at different ambient pressures and stored in the memory (26) and in that the evaluation device (19) is designed to additionally select or calculate the ambient reference spectrum (27, 27 _2f ) to be subtracted from the currently detected absorption spectrum (18, 18 _2f ) of the process gas mixture (2) as a function of the currently measured ambient pressure , Process gas analyzer according to one of the preceding claims, characterized in that the control device (12, 13, 15, 17) is additionally designed to the current-time function (i (t)) for driving the laser diode (7) with a sinusoidal Signal (16) predetermined frequency (f) and modulation amplitude to modulate, and that the evaluation device (19) is further adapted to the obtained by the detection of the light (9) ambient reference spectra (27) and the current absorption spectrum (18) of the Demodulate process gas mixture (2) at the predetermined frequency (f) or one of their higher harmonics (2f), wherein the ambient reference spectra (27) in the form of demodulated environmental reference spectra (27 _2f ) are stored, from which a demodulated ambient reference spectrum (27 _2f ) is selected or calculated and subtracted from the currently detected demodulated absorption spectrum (18 _2f ) of the process gas mixture (2). Process gas analyzer according to claim 5, characterized in that the evaluation device (19) is adapted to fit a parameter describable profile of an ideal demodulated absorption line to the selected oxygen absorption line in the demodulated environmental reference spectra (27 _2f ) and the demodulated environment Reference spectra (27 _2f ) in the form of the fitted profiles of the ideal demodulated absorption line, from which a profile is selected or calculated and subtracted from the currently detected demodulated absorption spectrum (18 _2f ) of the process gas mixture (2). A process gas analyzer according to claim 6, characterized in that the parameters describing the profile of the ideal demodulated absorption line comprise at least one modulation-specific parameter, and in that the evaluation device (19) is further configured to match the profile of the ideal demodulated absorption line to a currently detected one to pick different absorption line of the selected oxygen absorption line and the value of the at least one modulation specific Take over parameters for the corresponding modulation-specific parameter of the profile to be subtracted from the currently detected demodulated absorption spectrum (18 _2f ) of the process gas mixture (2).

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